Summary. Healthy subjects of African ancestry, including Afro-Caribbeans, have been observed to have lower total white cell counts and neutrophil counts than healthy Caucasian subjects. The cause of this ethnic neutropenia is unclear. We have previously found no evidence that increased margination of neutrophils is responsible. In this study, we have investigated mobilization of neutrophils from the bone marrow granulocyte reserve by endurance exercise. We investigated subjects of different ethnic origins before and after they had competed in a marathon race. Before the race, the neutrophil counts of Africans/Afro-Caribbeans were signi®cantly lower than those of Caucasians (means 2´49 and 3´21´10 9 /l respectively; P 0´043). After the race, the difference was greater, as was the degree of signi®cance (means 10´21 and 12´33 respectively; P 0´017). The percentage increment was almost identical whereas the absolute increment was considerably less, although of marginal statistical signi®cance (increment 7´72 and 9´12´10 9 /l respectively; P 0´10). Similar changes were observed in the monocyte count. Before the race, the difference in the means was of marginal statistical signi®-cance (0´35 and 0´41´10 9 /l respectively; P 0´105), whereas after the race the difference was greater and was highly signi®cant (means 0´75 and 1´05´10 9 /l respectively; P 0´001). These observations support the results of our earlier study, both of which suggested that ethnic neutropenia is likely to result from a diminished bone marrow reserve rather than being consequent on altered distribution of neutrophils within the blood stream.

This study was designed to determine whether ethnic neutropenia is caused by an increased proportion of neutrophils being present in the marginated granulocyte pool. Thirty two healthy volunteers, half of whom were African or AfroCaribbean and half of whom were white, exercised vigorously for 10 minutes on a step machine to mobilise granulocytes from the marginated granulocyte pool into the circulating granulocyte pool. The amount of work performed and the pulse rate response of the two ethnic groups were compared to determine whether the exercise carried out was comparable. A full blood count and an automated diVerential count were performed before and after the exercise. The haemoglobin concentration, platelet count, and absolute counts of total leucocytes and leucocyte subsets before and after exercise were compared in each individual and the values in the two ethnic groups both before and after exercise were compared. The absolute increase in neutrophils in the two ethnic groups was compared. The African/Afro-Caribbean group was found to have a reduced rather than enhanced ability to mobilise neutrophils from the marginated granulocyte pool. Therefore, increased margination of neutrophils is unlikely to be the cause of ethnic neutropenia. (J Clin Pathol 2000;53:481-483)

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